422 THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



head Neck, which resemble those of Cape Ann, are seen to cut the still older 

 porphyries." (Proc. Bost. Soc. Nat. Hist., 1875, XVII., pp. 508-510.) 



In 187G Mr. T. T. Bouv6 called attention to his views regarding the 

 derivation of the felsites in the vicinity of Boston from the metamor- 

 phosis of the conglomerates. He remarked that, in 1870, 



" Professor Niles distinctly stated that he had traced in Dedham the con- 

 glomerate until it passed into porphyry. He had noticed the effects of meta- 

 morphism where dikes occurred, and he believed that many of our porphyritic 

 rocks were formed from the conglomerate. These views I sustained by refer- 

 ring to my own observations, expressing myself satisfied that the porphyries 

 of our vicinity, as well as the amygdaloids, were altered conglomerates. Dr. 

 Hunt closed the discussion by saying he was confident that at Marblehead 

 these rocks were not altered conglomerates. They were derived rocks, but 



from the primitive parent rock on which they rested In conclusion I 



wish not only to re-express my belief in the derivation of these felsites from 

 conglomerates, but to go one step further, and include among the rocks having 



the same origin, some at least of the underlying sienites But I refrain 



from expressing more on this point, simply because my own observations in 

 the field have been so limited, but will ask if the reputed succession of our 

 rock deposits is not itself very suggestive. 



" Conglomerate. 



"Compact Feldspar, gradually passing into Porphyry. 



" Porphyry gradually passing into a rock intermediate between Porphyry 

 and Sienite. 



" Rock intermediate between Porphyry and Sienite. 



" Sienite. 



" Now if this gives the true succession of our rocks, .... I ask if it be not 

 a fair inference that the causes that led to the changes in the higher portions 

 of the series, affected all, only to a much greater degree the lower ; that the 

 heat and ac^ueous menstruum that softened and partly changed some of the 

 conglomerates of the upper portion forming the felsite conglomerate, .... 

 and which melted the succeeding strata so as to produce first felsites without 

 crystals, and below these the true porphyries, may not also by its greater 

 intensity so thoroughly have melted down still lower strata of sedimentary 

 rocks (conglomerates and slates perhaps), as to entirely resolve them into their 

 original elements, recrystallize them and thus have formed sienites, some of 

 which may have even subsequently played the role of eruptive rocks : for it by 

 no means follows that because a rock has been sedimentary that it may not 

 also have become likewise eruptive by being forced upward when in a senu- 

 fluid state." (Proc. Bost. Soc. Nat. Hist., 1876, XVIIL, pp. 217-220.) 



Prof. Hyatt, in support of Mr. Bouv6's views, remarked in regard to 

 the felsite of Marblehead Neck : 



